The present invention relates to a continuous process of making glass wherein batch is fed to a furnace equipped with heating means for melting the batch and with electrodes between which a direct electric current is established within the melt, to apparatus for performing the process, and to glass made using the process or the apparatus.
As is well known in the glass manufacturing art, a vitrifiable batch of the desired composition is introduced into the charging end of a tank furnace where it is melted in a melting zone and passes downstream in the tank through mixing and fining zones to a conditioning zone where the molten glass is allowed to cool to a desired working temperature. From the conditioning zone molten glass is passed to glass forming apparatus, for example a float tank or drawing machine or a bottle forming apparatus, where the glass is formed into a desired shape and allowed to cool. The expressions "upstream" and "downstream" are used herein to denote directions respectively towards the charging end and towards the conditioning zone of the tank.
It is well known that there is a region along the length of a melting tank where the atmosphere in contact with the glass is hottest. This is called the hot spot. From this point surface currents of glass must flow outwardly in all directions over a region which we call the spring zone. To maintain continuity there must be a current rising from the depths of the molten glass in the tank. This rising current can be considered as flowing from a particular point in the bath which we call the source. When no glass is being withdrawn from the tank and no batch is entering, the hot spot is at the center of the spring zone and is vertically above the source.
Diverging surface currents upstream of the spring zone travel towards the charging end wall where they tend to cool and form a falling convection current which returns along the bottom of the tank towards the source. Diverging downstream surface currents move along the tank, cool, and sink towards the bottom of the tank, and also return to the source.
Glass which flows into the conditioning zone comes largely from these downstream flowing surface currents, and, because it is hotter than the glass in the conditioning zone a further convection current circulation is set up there which causes a return flow of cooler glass from the conditioning zone along the bottom of the melting tank towards the source. This of course has an effect on the circulation of the glass in the regions of the source and spring zone. In fact the source may move slightly upstream so that its center is no longer coincident with the hot spot.
As the batch is delivered to the melting tank, it floats on the molten glass already in the tank in a layer which diminishes in thickness in the downstream direction. The downstream end portion of this layer of unmelted batch is generally covered with foam caused by the escape of gas bubbles as the batch constituents melt and react with each other and with the previously melted glass. This foam extends further downstream than the floating unmelted batch, but does not extend beyond a foam limit line because of the upstream-flowing surface currents of glass which diverge from the spring zone. This foam limit is of course upstream of the center of the spring zone. For the purposes of this specification, the spring zone is considered to be that area which is bounded at its upstream edge by the foam limit and has as its center that point on the surface of the bath from which surface currents diverge. Also the melting zone of the tank is defined for the purposes of this specification as the zone of the tank upstream of the center of the spring zone.
It is known to establish an electric current in the molten glass in a tank furnace for various purposes. One reason is to supply additional heat energy to the melt to raise its temperature. Another reason is to produce in the molten glass a stream of electrolytically formed oxygen bubbles so as to set up a particular desired pattern of flow currents in the molten glass in the tank to ensure that the melt is thoroughly mixed. If an anode producing such a stream of oxygen bubbles is located on the floor of the tank beneath the hot spot, the stream of oxygen bubbles may tend to stabilize the position of the source.